Failed to execute `SET TRANSACTION ISOLATION` in postgresql transaction

## Bug Report
### Which version of ShardingSphere did you use?
Master.

### Which project did you use? ShardingSphere-JDBC or ShardingSphere-Proxy?
Proxy.

### Expected behavior
Execute sql successfully.

### Actual behavior
Failed to execute sql in transaction

### Reason analyze (If you can)

### Steps to reproduce the behavior, such as: SQL to execute, sharding rule configuration, when exception occur etc.
Execute in ShardingSphere-Proxy
```sql
begin;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ, READ ONLY;
-- failed
```

Execute in postgresql
```sql
postgres=# begin;
BEGIN
postgres=*# SET TRANSACTION ISOLATION LEVEL REPEATABLE READ, READ ONLY;
SET
postgres=*# rollback;
ROLLBACK
postgres=# SET TRANSACTION ISOLATION LEVEL REPEATABLE READ, READ ONLY;
WARNING: SET TRANSACTION can only be used in transaction blocks
SET
```
### Example codes for reproduce this issue (such as a github link).